The Crystal Structures of Pb₅Sb₄S₁₁ (Boulangerite) – A Phase Transition Explains Seemingly Contradictory Structure Models

Abstract

Abstract. In the literature, many seemingly contradictory structure models of boulangerite, ideally Pb 5 Sb 4 S 11 , and the related mineral falkmanite have been reported. These can be explained by a phase transition between a low-temperature (LT) modification (P2 1 /c, a ≈ 8.07 Å, b ≈ 23.5 Å, c ≈ 21.6 Å, β ≈ 100.7°) and a disordered hightemperature (HT) modification [Pnma, a = 23.530(2) Å; b = 4.0385(8) Å and c = 21.273(2) Å, R 1 (obs) = 0.039] that occurs above ca. 350-400°C. The almost completely ordered distribution of lead and antimony cations in the LT modification involves a superstructure; the corresponding reflections are obvious in electron and X-ray diffraction patterns. Partial ordering may render them more or less diffuse. They vanish upon heating above the transition temperature, also for originally ordered natural boulangerite (from Trepča, Kosovo), and appear again after annealing below the phase transition temperature (e.g. at 330°C). The HT phase is characterized by pronounced cation disorder. Cation ordering can be described by group-subgroup relationships which may also serve for a unified description of various structure models from literature. Starting from the orthorhombic HT phase, a

Cornucopia of Structures in the Pseudobinary System (SnSe)xBi2Se3: A Crystal-Chemical Copycat

“…250 °C may indicate an alteration of the structure or the composition, for example, due to exsolution processes or an order−disorder transition as observed for Pb 5 Sb 4 S 11 . 71 Subsequent SEM and TEM studies on the sample revealed a decomposition of the sample into a Sn-enriched phase with L4,4-type structure (space group Pnma) and small amounts of a Bi-enriched unknown phase (see Figure S13 and Table S15 in the Supporting Information). The phase stability was not further investigated due to the poor thermoelectric properties.…”